GRB 000418: A Hidden Jet Revealed

We report on optical, near-infrared, and centimeter radio observations of GRB 000418 that allow us to follow the evolution of the afterglow from 2 to 200 days after the γ-ray burst (GRB). In modeling these broadband data, we find that an isotropic explosion in a constant-density medium is unable to simultaneously fit both the radio and optical data. However, a jetlike outflow into either a constant density or wind-stratified medium with an opening angle of 10°-20° provides a good description of the data. The evidence in favor of a jet interpretation is based on the behavior of the radio light curves, since the expected jet break is masked at optical wavelengths by the light of the host galaxy. We also find evidence for extinction, presumably arising from within the host galaxy, with A = 0.4 mag, as well as host flux densities of FR = 1.1 μJy and FK = 1.7 μJy. These values supersede previous work on this burst as a result of the availability of a broadband data set allowing a global fitting approach. A model in which the GRB explodes isotropically into a wind-stratified circumburst medium cannot be ruled out by these data. However, in examining a sample of other bursts (e.g., GRB 990510, GRB 000301C), we favor the jet interpretation for GRB 000418.